This invention pertains in general to a semiconductor integrated circuit, and more particularly relates to an improved method of isolating active devices.
In integrated circuit technology, it is necessary to separate the active regions containing active devices from one another. In early bipolar integrated circuits, the active regions were generally electrically isolated from each other by PN junctions. However, with increasing demand for higher device densities, it has become necessary to reduce the isolation areas.
In VLSI integrated circuits using MOS technology, isolation of active regions has usually been accomplished by LOCOS (Local Oxidation of Silicon). To perform LOCOS, a patterned nitride on top of a thin oxide stack is used to cover what will be active areas of a silicon substrate. By exposing the uncovered regions of the silicon substrate to a high temperature oxidizing ambient, a relatively thick field oxide is grown only in the exposed regions.
However, the LOCOS technique grows field oxide not only vertically in the exposed silicon regions, but also laterally underneath the edges of the nitride mask. This lateral oxide encroachment under the nitride, known as "birds-beak," can extend laterally to a distance of about half the field oxide thickness; thus, substantial real estate is wasted in this isolation technology. With the standard LOCOS process, the field oxide thickness has to be scaled down appropriately in order to reduce the birds-beak, otherwise, the remaining active areas will be inadequate for forming active devices. The reduction in field oxide thickness, however, degrades the circuit performance because of increased interconnect capacitance. In addition, the leakage current under the field oxide and between adjacent active areas increases rapidly with decreasing oxide thickness for a given voltage applied to a conductor passing over the field oxide, resulting in reduced isolation between adjacent active areas.
Further problems are encountered when using LOCOS to form wide and narrow isolation regions simultaneously. In particular, since the growth of field oxide is vertical and lateral, the thickness of grown field oxide in narrow regions can be substantially less than in wide regions. Thus, to achieve a desired field oxide thickness in narrow regions, a much larger field oxide is grown in wide regions with concomitant much larger "birds-beak".
Isolation between two adjacent components of the integrated circuit can also be achieved with the use of minimum surface area by etching a trench extending into the substrate between the two components and then refilling the trench with an insulator. Trench isolation uses much less surface area than does either diffused junction isolation or local oxide isolation.
High quality integrated circuits require the semiconductor material along the edge of the trench to be of high integrity with a minimum of process induced defects. Reactive ion etching, which is a preferred method for anisotropically etching trenches, has a tendency to produce a thin defect layer along the trench walls. Defects of this type degrade device performance by forming parasitic channels along the trench to active region junction which increase leakage currents.
To reduce the formation of such a parasitic channel along the trench, the insulating layer directly adjacent to the trench edges is desired to be a thermally grown silicon dioxide layer.